Automatic Doppler gate positioning in spectral Doppler ultrasound imaging

ABSTRACT

A Doppler gate is automatically positioned in spectral Doppler ultrasound imaging. Samples acquired for multiple PW Doppler gates are used for B-mode and/or F-mode detection over time without interleaving transmissions for the PW Doppler. The B-mode and/or F-mode information are used to track gate placement. Alternatively or additionally, characteristics spectra from different gate locations are used to select a gate location. Either tracking may be used to change the locations sampled and/or beam characteristics, such as centering the locations and beam focus on the selected gate location.

RELATED CASE

This application is a continuation of U.S. application Ser. No.13/548,561, filed Jul. 13, 2012, which is hereby incorporated byreference in its entirety.

BACKGROUND

The present invention relates to pulsed wave (PW) spectral Dopplerultrasound. Spectral Doppler ultrasound imaging provides an image ofvelocities (vertical axis) values modulated by energy as a function oftime (horizontal axis). This spectrum may be used for studying fluidflow or tissue motion within a patient. By transmitting a plurality ofpulses at a single gate location, a spectral Doppler response isgenerated in response to received echo signals. The frequency spectrumof the object's motion or flow for a single spatial region is estimatedand displayed as a function of time.

Sonographers manually adjust the gate location, gate size, transmitfrequency and other spectral Doppler imaging control parameters in orderto acquire a desirable image. The gate placement is assisted by displayof a 2D B-mode image of the anatomy of interest. Some processes havebeen proposed for automatic placement of the spectral Doppler gate usingB-mode or color Doppler (F-mode) information. However, obtaining theB-mode or F-mode information interrupts the acquisition of therelatively high pulse repetition frequency PW Doppler. A briefinterruption (e.g., 10-20 ms) allows at least a portion of the two orthree-dimensional B-mode or F-mode data to be acquired. This producesgaps in the PW Doppler measurement. Depending on the temporalcharacteristics of the PW Doppler waveform due to flow dynamics, vitalinformation may be lost during this time interval.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, systems, computer readable media, and instructions forpositioning a Doppler gate in spectral Doppler ultrasound imaging.Samples acquired for multiple PW Doppler gates are used for B-modeand/or F-mode detection over time without interleaving transmissionsfrom the PW Doppler. The B-mode and/or F-mode information are used totrack gate placement. Alternatively or additionally, characteristics ofspectra from different gate locations are used to select a gatelocation. Either positioning may be used to change the locations sampledand/or beam characteristics, such as centering the locations and beamfocus on the selected gate location.

In a first aspect, a method is provided for positioning a Doppler gatein spectral Doppler ultrasound imaging. Transmit beams are transmittedrepetitively from a transducer array. In response to the transmitting,signals from different receive locations are repetitively received.B-mode information representing the different receive locations atdifferent times is detected from at least some of the signals. TheDoppler gate location is tracked with the B-mode information. A firstspectrum for the tracked Doppler gate location is estimated from thesignals for the tracked Doppler gate location. An image is displayed asa function of the first spectrum.

In a second aspect, a non-transitory computer readable storage mediumhas stored therein data representing instructions executable by aprogrammed processor for positioning a Doppler gate in spectral Dopplerultrasound imaging. The storage medium includes instructions forreceiving signals over time for each a plurality of spaced apartlocations, performing spectral analysis of the signals separately foreach of the spaced apart locations, the spectral analysis providingspectra for each of the spaced apart locations, detecting acharacteristic of each spectra from the spaced apart locations, settinga Doppler gate location to one of the spaced apart locations as afunction of the characteristic of each spectrum, and updating adistribution of the spaced apart locations as a function of the setDoppler gate location so that a center of the distribution is at the setDoppler gate location.

In a third aspect, a system is provided for positioning a Doppler gatein spectral Doppler ultrasound imaging. A transmit beamformer isoperable to transmit beams. A receive beamformer is operable to form aplurality of spaced apart receive beams in response to each of thetransmit beams, each of the receive beams sampled at a plurality ofdepths. A processor is configured to set a location of the Doppler gateas a function of the sampled receive beams and to control the transmitbeamformer to center the transmit beams and spacing of the sampling ofthe receive beams on the location.

In a fourth aspect, a non-transitory computer readable storage mediumhas stored therein data representing instructions executable by aprogrammed processor for positioning a Doppler gate in spectral Dopplerultrasound imaging. The storage medium includes instructions forreceiving signals over time for each of a plurality of spaced apartlocations, detecting B-mode information for different times from atleast some of the signals, tracking a location over time with the B-modeinformation, updating the spaced apart locations as a function of thetracked location, performing spectral analysis of the signals separatelyfor each of the spaced apart locations, the spectral analysis providingspectra for each of the spaced apart locations, detecting acharacteristic of each spectra from the spaced apart locations, andsetting a Doppler gate location to one of the spaced apart locations asa function of the characteristic of each spectra.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a flow chart diagram of one embodiment of a method forpositioning a Doppler gate in spectral Doppler ultrasound imaging;

FIG. 2 is a graphical representation of an example spectrum;

FIG. 3 is a graphical representation of example parallel beamforming ina region of interest;

FIG. 4 is a graphical representation of an example spectral stripdisplay;

FIG. 5 is a graphical representation of an example constellation ofsample locations and a Doppler gate location in a region of interest atone time;

FIG. 6 is a graphical representation of the example constellation of thesample locations and the Doppler gate location of FIG. 5, but tracked toa different location for a different time; and

FIG. 7 is a block diagram of one embodiment of a system for positioninga Doppler gate in spectral Doppler ultrasound imaging.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Automatic Doppler gate placement is provided in ultrasonic blood flowmeasurement, such as for cardiac valve applications, or in tissue motionmeasurement. Once the gate has been placed, continuous positioningensures that the gate position is maintained even in the presence ofmotion from the patient (e.g., from breathing) or the transducerposition (e.g., from the sonographer moving). The continuous monitoringand/or positioning of the gate occurs without interrupting PW spectralDoppler acquisition for other types of scanning.

Multi-transmit and/or receive beam capability is used to acquire aconstellation of Doppler gates in a region near the optimal gateposition. PW Doppler gate position and/or beamforming are based onsimultaneous acquisition of data for multiple spectral Doppler gates.Multiple gates may be located along an ultrasound beam (multiple rangegates). Parallel receive beamforming allows multiple gates in lateraland/or elevation directions within the area covered by a transmit beamor beams. Using split beam (parallel transmit beams), two or morespatially distinct regions may be interrogated.

The data acquired for the constellation represents the region orregions. The data acquired at the Doppler gates may also be used forB-mode and/or F-mode detection. The spectral Doppler signals themselvesand/or the B-mode/color region of the constellation are used forpositioning. As the target moves because of patient or transducermotion, the optimal gate placement is maintained. The detection andtracking of motion only requires a constellation of Doppler gates to beacquired without interrupting the PW Doppler acquisition and whilemaintaining the highest temporal sampling quality.

In one embodiment, the multigate constellation acquisition provides datafor concurrently acquired B-mode and/or F-mode images in addition to thePW Doppler information. The B-mode and/or F-mode images are used forcontinuous tracking. The optimal gate position is maintained byadjusting the location used for estimating spectra.

In another embodiment, a Doppler strip is generated for each location ofthe multigate constellation. Each strip is analyzed and compared withthe others to determine the best gate position.

Both approaches may be used together. For example, the B-mode trackingis used to center the constellation of gates. The spectral analysis isused to select the Doppler gate location from among the constellation ofgates.

In either approach, lateral and/or elevation transmit beam profiles andtransmit focus depth may be adjusted to be centered at the optimal gateposition. The constellation may be adjusted to be centered at theoptimal gate position. The transmit line and/or transmit focal depth ofthe center of the gate constellation is moved in response to motion. Thehighest transmit power and best beam profile characteristics may becentered on a region of acquired Doppler gates, maintaining the bestcharacteristics while tracking motion.

Where there is loss of tracking or excessive movement is detected, thesystem may reset. A new image-based gate placement cycle isautomatically initiated instead of continuing to track or position.

FIG. 1 shows method for positioning a Doppler gate in spectral Dopplerultrasound imaging. The method is implemented on the system 10 of FIG. 7or a different system. The acts are performed in the order shown, butother orders are possible. Acts 30 and 32 are performed sequentially orsimultaneously with acts 34 and 36. Acts 38 and 40 are performedsimultaneously or sequentially in any order. Act 42 may be performedbefore act 40. Other orders may be used.

Additional, different, or fewer acts may be provided. For example, acts30 and 32 are not performed. As another example, acts 34 and 36 are notperformed. In yet another example, act 46 is not performed. Acts 40and/or 44 may not be performed. Various combinations may be used.

For positioning a PW Doppler gate, ultrasound samples or signals areobtained for a plurality of spatially distinct locations. The samplesare obtained by transmitting beams in act 26. One or more transmit beamsare transmitted at a given time. To cover a larger region, simultaneoustransmit beams may be formed. Simultaneous formation of beams ondifferent scan lines may be used. For example, two transmit beams areformed at different steering angles, from different origins on thetransducer array, and/or from the transducer at different positions. Thetransmit beams are formed along different scan lines. In the near fieldand far field, the transmit beams may over lap. At the focal region someor no overlap is provided. The −6 dB or −10 dB edge of the transmitbeams overlap or are separated by a region of lesser acoustic power fromthe transmit beams. Non-overlapping regions in the middle field, farfield and/or near field may be used.

Two or more beams are transmitted substantially simultaneously.“Substantially” accounts for different delays or start of transmissionsdue to different foci or steering. “Substantially” provides for twobeams to be transmitted within sufficient time of each other that atleast a portion of a wavefront of one waveform is generated acousticallybefore the last of returned echoes for another wavefront are received atthe transducer. The wavefronts from both beams may be transmitted by amajority of the elements of the transducer prior to any receptionoperation. Simultaneous transmission includes generating acousticwaveforms for one beam while also doing so for another beam, such astransmitting a waveform for one beam from one element while alsotransmitting a waveform for another beam from another element or the oneelement.

The split beams (e.g., substantially simultaneous transmit beams alongdifferent scan lines) are generated using any possible method. Forexample, different apertures are formed on the transducer array. Eachaperture is for transmitting a different one or ones of the transmitbeams. The apertures are unique or do not overlap, such as using rightand left halves of the array for two different beams. The apertures maybe neighboring sections, may be spatially interleaved (e.g., every otherelement for one aperture and the other elements for another aperture),or may overlap (e.g., one or more elements transmit waveforms for bothbeams). The different apertures produce spatially distinct transmitbeams by application of a suitable delay and/or phasing pattern.

In another embodiment, the waveforms for two or more beams are appliedto the same or overlapping apertures at a substantially simultaneoustime. For each element, the electrical waveforms for the different beamsare combined (e.g., summed) based on the separate delay and/or phasingand apodization profiles. The combined waveforms are transmitted fromthe elements of the aperture, forming the transmit beams substantiallysimultaneously.

For receive isolation or to limit contribution to received samples forone beam from another beam, different center frequencies, coding, orcenter frequencies and coding may be used for each beam. For example,frequency multiplexing is used. Two or more transmit pulses havingdifferent center frequencies are transmitted. Different delay profilesare used for the pulses at different frequencies so that two or morebeams are created in parallel (temporally). For coding, any coding maybe used, such as spread spectrum codes or orthogonal codes.Frequency-based codes, amplitude-based codes, phase-based codes, orcombinations thereof may be used. In alternative embodiments, no codingor frequency different is provided. The spatial differences in thetransmit beams differentiate the receive signals.

In other embodiments, combinations of techniques for generatingsubstantially simultaneous transmit beams may be used. For example, theaperture is split into two spatially overlapping groups. The groupstransmit pulses having different center frequencies so that spatiallydistinct beams are generated that are separated in frequency.

In other embodiments, one transmit beam is formed at a given time. Thetransmit beam is formed with a sufficient beam width to allow receivebeamformation along laterally and/or elevationally spaced receive scanlines. Plane wave, infinite focus, spread beam, or narrow beam withsufficient width may be used. Split or multi-beam may be used with suchwide or diverging wavefront transmit beams.

For either the simultaneous beams or the single beam, the transmissionsare repeated. The repetition allows reception of sufficient samples toperform spectral analysis.

In act 28, signals for a plurality of laterally spaced locations arereceived in response to each of the transmit beams. Receive beams areformed along a plurality (e.g., two or more, such as 32 or 64) of scanlines in response to each of the transmit beams. A plurality of scanlines and ranges along the scan lines may be sampled in response to thesingle transmission. Parallel receive beamforming is provided. Theultrasound samples are obtained at a substantially same time along aplurality of receive beams responsive to a same transmit beam. Otherplane wave transmission and reception techniques may be used, such asapplying a Fourier transform to electrical signals at each element togenerate an array of values representing response at differentlocations.

FIG. 3 shows one embodiment where one transmission of split beams in act26 is used to acquire a plurality of receive beams in act 28. While tworeceive beams are shown, a greater density may be provided, such asfour, eight, sixteen, thirty two, sixty-four, or other number of receivebeams per transmit beam. The transmit and receive beams intersect aregion of interest.

The region of interest may be any size or shape. The region of interestdefines the spatial locations for which spectra may be estimated. Forexample, at least one hundred locations are sampled for possiblespectral analysis. The region may be contiguous or divided. Multipleregions may be scanned.

Any sampling density of locations may be used in the region of interest.The distribution of locations is a constellation of sample points forpossible spectral analysis. The constellation may be a distribution intwo or three dimensions. Symmetric or asymmetric distribution may beused, such as sampling in 64 lateral and elevation spaced locations andat 10 depth spaced locations.

The receive operation occurs repetitively in response to thetransmitting. Signals from laterally and/or elevationally distinctreceive locations within the transmit beams are received. By forming aplurality of receive beams in response to each of the transmit beams,signals for many receive locations are obtained substantiallysimultaneously. “Substantially” accounts for the acoustic travel timealong a line in a field of view.

Samples for the same locations are acquired over time. Ultrasoundsamples are obtained over a period, such as acquiring five or moreultrasound samples for each spatial location. Any scan sequence and/orpulse repetition frequency may be used.

The PW Doppler gate is to be positioned at one of the sample locationsof the constellation. Sufficient samples are obtained to estimate thespectra over time for any of the sample locations. Three differentapproaches may be used alone or in combination to position the PWDoppler gate for a spectral strip display at one of the samplelocations. Acts 30 and 32 represent an approach using B-mode detection.Acts 34 and 36 represent an approach using spectral analysis. Act 46represents an approach using F-mode detection. Other approaches may beused.

In act 30, B-mode information is generated. The B-mode information isgenerated from the PW Doppler samples. While the transmit and/or receivecharacteristics (e.g., frequency, number of cycles, F#, or aperture) maytypically be different for PW Doppler and B-mode, samples acquired forPW Doppler may be used for B-mode detection. The transmit and/or receivecharacteristics may be compromised for both spectral analysis and B-modedetection or optimized for B-mode detection in other embodiments. Thesame data is used for both.

For spectral analysis, an ensemble of signals from a same location isacquired, such as five to twenty samples for each spectrum. The samplesmay be obtained in an ongoing manner such that a moving window (e.g.,ensemble or flow sample count) with any step size (e.g., every sample orevery third sample) is used to estimate a spectrum. The B-mode detectionuses a single sample to estimate the intensity. One of the samples froma given ensemble is selected and used. To estimate B-mode at differenttimes, signals from different times in the same or different ensembleare selected. B-mode information may be detected for each signal for thegreatest temporal resolution. In other embodiments, less than all of thesignals are used for B-mode detection, such as performing B-modedetection with every fifth sample.

Since signals are acquired for the constellation, the B-mode data isdetected for the region of interest. The intensity from single samplesof the signals for the different receive locations is detected. Thedetection is performed for different times. B-mode information generallyrepresents return from tissue or other structure within the patient. Bydetecting over time, the tissue in the region of interest is detected atdifferent times. As motion occurs, the tissue appears or does shift,rotate, compress, or expand. The B-mode information over time reflectsthe change.

In act 32, a location is tracked over time with the B-mode information.The location is a Doppler gate location. For example, the user places aninitial Doppler gate. As another example, act 46 or acts 34 and 36 areused to initially place the Doppler gate. In another example, automaticplacement using a prior B-mode scan (e.g., boundary or edge detection toplace a gate in the center of an enclosed boundary) or other approach isused.

The location of the Doppler gate is tracked. In other embodiments, thelocation being tracked is the entire region of interest. Other locationsmay be tracked, such as a center of the region of interest or a sub-areaor sub-volume of the region of interest.

The tracking detects a change of the location over time. The locationmay be at one coordinate at one time, but shift to be at anothercoordinate at another time. The coordinates are defined with respect tothe scanning, such as with respect to the scan format from theultrasound transducer. The location is relative to the patient. Data atdifferent coordinates for different times may represent the samelocation.

For tracking the location, a kernel of B-mode information representingdifferent spatial locations is used. Any size kernel may be provided,such as 9×9 or 12×12×12 neighborhood around the location or the entireregion of interest. The entire region of interest may be used as thekernel.

The kernel is a reference set. The reference is for B-mode informationat a first, selected, or given time. The reference may be updated, suchas changing the reference over time. For example, the reference B-modeinformation is temporally adjacent to the most recently acquired B-modeinformation in a moving window. The reference B-mode information isupdated each time another frame of B-mode information is detected. Inanother embodiment, the reference B-mode information is only updatedonce sufficient motion has occurred to change the constellation locationand/or beam location (e.g., scan line shift) in act 40.

To track, the reference information is compared to the B-modeinformation from another time. Multiple comparisons are made between theB-mode information from two times. Different translation, rotations,and/or scales are attempted. The translation, rotation, and/or scalewith the best or highest similarity indicate the motion or change inposition between times. In one embodiment, just translation is tracked.

Any measure of similarity may be used. For example, a minimum sum ofabsolute differences is calculated. Cross-correlation or other measuresmay be used.

In another approach to determine positions for the Doppler gate overtime, spectra are estimated for the different locations in act 34.Spectra are estimated for the receive locations. A spectrum is estimatedfor each of the spatially distinct locations. The spectra are estimatedfrom the ultrasound samples from different depths, elevation, and/orlateral locations. The spectra correspond to a period in which thesamples were acquired. For each spatial location of interest, such asall the locations in a region of interest, in an image field, or otherdistributions, a spectrum is calculated. Spectra may be determined foronly a subset of the spatial locations, such as determining the spectrafor sparsely sampled locations or densely sampled locations but in alimited region.

For each receive location, a spectrum or spectra are estimated from thereceived signals. The spectrum is estimated by applying a Fouriertransform, wavelet transform or Wigner-Ville distribution to theultrasound samples representing each of the spatially distinctlocations. Spatially distinct locations correspond to different rangegates, such as different center positions, sizes or both, with orwithout overlap. Any transform may be applied to determine the spectrumfor each of the spatially distinct locations. Each spectrum representsenergy as a function of frequency (see FIG. 2).

Multiple spectra are estimated for each of the locations. FIG. 4 shows aspectral strip of spectra for a same location over time. Differentspectra may be estimated for the same spatial location at differenttimes corresponding to different periods or ensembles of acquisition.The spectrum for a given time is mapped with velocity on the horizontalaxis and energy modulating the intensity. Other mapping may be used. Thespectra are estimated, but may or may not be displayed.

The spectral analysis of the signals is performed separately for each ofthe spaced apart locations. The signals for each given location are usedfor spectral analysis without signals from other locations. Inalternative embodiments, the signals are spatially and/or temporallyfiltered prior to spectral analysis, but separate spectra are providedfor each location.

A set of spectra for a given time or representing the sampling periodare estimated. In one embodiment, all of the spatial locations fordetermining spectra are sampled at a same time (e.g., same transmit andreceive events). The spectra are sampled at a same time relative to aphysiological cycle, such as the heart cycle. Spectra for only oneperiod may be estimated. Spectra for the same locations are estimatedfor different periods to provide time varying spectral information. Thesamples used for estimating the spectra at a given time may be used forestimation at another time as well, such as associated with repeatingestimations using a temporally moving window for selecting the samples.

In act 36, one or more characteristics of the spectra for each locationare detected. Any characteristic of the spectra may be used, such as themaximum velocity, minimum velocity, mean velocity, median velocity,maximum energy, velocity associated with maximum energy, intensity,variance of velocity, range of velocity, slope or trend in the spectra,location of change of slope, shape over time, similarity to a pattern orspectra template, clutter, signal-to-noise ratio, combination of energyand velocity, or phase shift relative to known or measured cycle.

The characteristic of the spectra may be derived from one spectrum aftercomparison with other spectra for the location. For example, the maximumvelocity over all of the spectra for a location is associated with onespectrum. A single spectrum of the spectra may be analyzed for thecharacteristic, such as using clutter from the first or last spectrum.The combination of spectra may be used, such as pattern matching atemplate of spectra with the spectra for the location.

Different types of spectral information may be useful for differentdiagnostic purposes. For example, the maximum velocity may moreaccurately indicate tissue health. The variance of the spectra mayindicate flow conditions. The useful information provided in spectralstrips is available for many locations at a same time, providing forselection of the spectra associated with the desired characteristics.Locating the spectrum or spectra with the maximum velocity may providebetter flow information than a user guessed position.

Any now known or later developed techniques may be used to characterizeor determine a characteristic of the spectra. For example, the highestvelocities above a threshold level with only one or no lower velocitiesbelow the threshold indicate the maximum velocity. The maximum velocityis the highest or an average of the two or more highest velocitiesassociated with contiguous values above the threshold or noise level inthe spectrum. The signal-to-noise ratio may be calculated by measuringenergy or brightness of the spectrum or spectra from samples with thetransmitter turned off and samples with the transmitter active. Asanother example, clutter may be measured based on mapping from velocityand energy, such as high energy with low velocity indicating strongerclutter strength. Clutter may be measured by a ratio or difference ofenergy with and without clutter filtering.

In act 46, F-mode information is estimated from the signals. F-mode iscolor Doppler or other spatially distributed estimate of mean velocity,energy, and/or variance. Using the same ensemble or a sub-set of theensemble acquired for spectral estimation, the F-mode information forthe different locations is estimated. F-mode information indicatescharacteristics of flow, such as showing a flow region with higher andlower flow locations.

In act 38, the Doppler gate location is set. One of the locations of theconstellation is selected as the Doppler gate location or the locationfor which spectra are to be displayed or output. More than one locationmay be selected, such as selecting a location on each side of a heartvalve so that spectral information is provided for both locations.

The location is set based on the tracked B-mode location, thecharacteristics of the spectra, or the F-mode information. For B-modetracking, the location for the tracked Doppler gate is used. As theinitial location alters position, the signals from the new position areused for spectral analysis. The tracking follows a feature or structureof the patient. At different times, the signals from differentcoordinates represent the response of the feature. By using the signalsfrom the tracked location over time, the resulting spectra represent thefeature despite motion.

Signals from different locations are used to estimate spectraseparately. As the tracked Doppler gate changes position, any newspectrum is determined for the new location. This spectrum is added tothe spectral strip or spectra from a previous location or locations.Alternatively or additionally, signals from different locations may becombined into an on-going stream, such as including signals fromdifferent locations in a given ensemble for estimation of a spectrum.

For setting the Doppler gate location based on the spectracharacteristics, the characteristics of the spectra for differentlocations are compared. For example, the best fit of a template to thespectra of the different locations is identified. As another example,the location that has the most amount of correct flow characteristics,such as velocity above a given level, and has the least amount ofundesired characteristics, such as clutter, is selected. Fuzzy logic,mapping, weighted averaging or other logic may be used to combine thevalues for different characteristics to select one location.Alternatively, the best gate location may just be the location with thehighest signal-to-noise ratio for high velocity signals whilediscounting gates with strong clutter signals.

In another approach, the Doppler gate location is set using F-modeinformation. The location associated with a center of gravity orgeometric center of a largest region of flow is identified. Flowcharacteristics associated with a valve or other object may beidentified and used to set the location. The location associated with agreatest velocity, energy, variance, or combination thereof may be used.

In one embodiment, the Doppler gate location is set based on acombination of approaches. Any combination of two or three of theB-mode, spectral analysis, and F-mode approaches may be used. Otherapproaches may be used in combination with one, two or all three of theB-mode, spectral analysis, and F-mode approaches. The combination may beby averaging locations output by each approach. The combination may beby selecting a location output by the different approaches based on acriterion, such as the location most similar to the other outputlocations (e.g., the middle location from three possibilities).

In one embodiment, the combination is a provisional setting by oneapproach and then refining using another approach. For example, theB-mode information is used for tracking the initial Doppler gate. Thetracking is used to establish the constellation of locations (see act 40below). The multi-gate constellation is acquired, and the spectralanalysis is used to select the Doppler gate location from theconstellation. The tracking is then performed based on the selectedDoppler gate location in a repetition of the process. This allows thepossibility that the optimal gate position may be time varyingthroughout the heart cycle.

In another example combination, the B-mode tracking is refined based onthe F-mode information. The B-mode tracking indicates the location ofthe constellation. The F-mode information for the tracked constellationis used to set the Doppler gate location, such as at the center of thedetected flow. Complex flow profiles found in diseased valves, such asdue to calcification, may have both positive and negative flow (i.e.,flow towards and away from the transducer). F-mode information may beused to properly place the Doppler gate as desired in complex flowprofiles.

In act 42, the spectra for the positioned Doppler gate are selected orestimated. The location or locations set in act 38 define the signalsused for spectral Doppler. As the locations vary over time, the signalsused for the spectral Doppler are based on the old, new or old and newlocations. Where the spectra are already estimated, such as in act 34,the spectra may be selected (e.g., loaded from memory). Alternatively,the spectra are calculated again with the same or different estimationsettings (e.g., flow sample count or ensemble size or with samples froma combination of locations being used in one ensemble). Where thespectra have not been previously estimated for setting the Doppler gatelocation, estimation is performed.

In act 44, an image is displayed. The image is a function of at leastone of the spectra for the plurality of spatially distinct locations.The spectra are used to provide information to the user. The image mayprovide information associated with only one spectrum in otherembodiments.

In one embodiment, a spectral strip for the Doppler gate location isdisplayed. FIG. 4 shows an example spectral strip display simplified forillustration. The spectral strip shows the frequency modulated by energyas a function of time. Any now known or later developed spectral stripmapping may be used, such as gray scale mapping with the intensityrepresenting energy. Filtering may be applied to smooth the spectrum.Characteristics of the spectral strip may be determined and displayed,such as graphically tracking a maximum velocity as a function of time inthe spectral strip.

Since the location or coordinates for the Doppler gate may change overtime, the spectral strip is generated from signals for differentlocations. Different ones of the spectra may be estimated from signalsfor different locations. A given spectrum may be estimated from signalsfrom different locations. As the Doppler gate position is set todifferent locations at different times, the spectral strip is displayedin an on-going manner as if representing a given Doppler gate location.

Multiple strips may be displayed. For example, spectral strips for twoor more selected locations are output for comparison. Each of themultiple selected Doppler gate locations is tracked or positioned overtime. The resulting multiple spectral strips provide spectra for thedesired feature of the patient.

In one embodiment, the spectral strip is displayed with a spatial image,such as a one-dimensional M-mode, two-dimensional B-mode,two-dimensional F-mode, or combination thereof image. The image is ofthe region of interest using data acquired for PW Doppler sampling. Thelocation of the selected spectrum or spectra may be indicatedgraphically in the image, such as represented by the circle in theregion of interest of the field of view shown in FIGS. 5 and 6. Forexample, text, color, symbol, or other indicator shows the user thelocation for the automatically determined range gate corresponding tothe selected spectrum. Where multiple spectra are displayed, matchedcolor coding between the acquisition range gates and displayed spectramay be used. For example, the indication of the location of the rangegate uses orange. The corresponding spectrum is shaded in orange,outlined in orange, or otherwise labeled in orange. Other indicationsmay be used, such as text labels or numbering.

In act 40, acts 26 and 28 are updated based on the setting of theposition of the Doppler gate. As the Doppler gate changes to differentcoordinates due to movement, the acquisition of data is changed.

In one embodiment, the transmit beam and/or receive beam position ischanged to be centered on the Doppler gate location. Any characteristicof the beam position may be set, such as the scan line origin, scan lineangle, or focus. For example, the scan line angle is changed laterallyor in elevation to cover the set Doppler gate. The transmit beam maycover multiple lateral and elevation locations. Since the energy may bestronger at the center of the transmit beam, the center or scan line ofthe transmit beam is changed to cover the current location of theDoppler gate. A receive beam is similarly positioned to intersect withthe current location of the set Doppler gate.

The focus may be changed to the Doppler gate. Where the Doppler gate isat a different depth, the focus is changed to the different depth.

Other characteristics of the beams may change by location. For example,the transmit beam may be made wider or narrower. The F#, apodization, oraperture may vary based on the location.

The updating of the beams may provide stronger signal-to-noise ratio forthe Doppler gate location than for other locations. Since the Dopplergate is used to output information, a stronger signal-to-noise ratio isdesired. Other locations are still sampled for positioning the Dopplergate. In alternative embodiments, the beams do not change based on thesetting of the Doppler gate.

In another embodiment, the constellation of spaced apart locations isestablished based on the set Doppler gate. The spatial sampling is forpositioning the Doppler gate. Given a previous Doppler gate, the spatialsampling is centered on the Doppler gate for optimizing the setting offuture gate locations. FIG. 5 shows a constellation of a region ofinterest represented by dots in a box. The constellation is centered onthe Doppler gate, located at the circle. Other locations than the centermay be keyed to the Doppler gate. The sampling distribution orconstellation is updated over time to be centered at the set Dopplergate. As the Doppler gate changes, the locations sampled also changes.FIG. 6 shows the location of the Doppler gate changing coordinatesrelative to the field of view. The constellation also changes. Inalternative embodiment, the sampled locations are static or do notchange based on the setting.

The sampling distribution may change based on the change in the beams.As the beams change location, the sampling locations likewise change. Inother embodiments, the sampling locations change while the beams aremaintained or vice versa.

Small excursions in the optimal Doppler gate position may not trigger achange in the acquisition of the signals. For example, the change inposition of the Doppler gate is compared to a threshold. Changes by oneor two location widths (or other distances) in the sampling distributiondo not trigger change in the acquisition, but larger changes do triggerchange.

The movement or change in the Doppler gate position may be used forother purposes. The process or acts 26-46 is ongoing based on an initialsetting of the Doppler gate. As motion occurs, the Doppler gate positionis updated. For large, rapid motion, the setting may not perform asdesired. In response to detecting a sufficiently large (above athreshold amount) motion, the process may be reinitiated. The Dopplergate is again set manually or initially before tracking or other settingupdate of the Doppler gate position. Gate placement is performed withoutthe B-mode tracking, F-mode setting, or spectral analysis. For example,if the signal from the constellation is completely lost or the trackingbased on B-mode is determined to be far off track (e.g., highestsimilarity is below a threshold), the system triggers a return to B-modeand/or F-mode acquisition (i.e., interleaved with or without PW Doppleracquisition) for an interval. The interleaved B-mode or F-mode scanningallows a new gate constellation position to be determined via imagebased techniques or manually. This triggering may alternatively oradditionally be based on an ECG waveform, so that the acquisition neededfor the position analysis is performed at a known part of the cardiaccycle.

FIG. 7 shows a system 10 for positioning a Doppler gate in spectralDoppler ultrasound imaging. The system 10 is a medical diagnosticultrasound imaging system, but other imaging systems may be used, suchas a workstation. The system 10 estimates spectra for a Doppler gatelocation and positions the Doppler gate location over time based on PWsampling without interleaving for B-mode or F-mode specific acquisition.The system 10 centers a constellation of sample locations and centers atransmit and/or receive beams on the Doppler gate. As the Doppler gateis set to a different location due to motion, the constellation and beamcenters are repositioned.

The system 10 includes a transmit beamformer 12, a transducer 14, areceive beamformer 16, an image processor 18, a display 20, and a memory22. Additional, different or fewer components may be provided, such asthe system 10 without the front-end beamformers 12, 16 and transducer 14or the system 10 with a scan converter.

The transducer 14 is an array of a plurality of elements. The elementsare piezoelectric or capacitive membrane elements. The array isconfigured as a one-dimensional array, a two-dimensional array, a 1.5Darray, a 1.25D array, a 1.75D array, an annular array, amultidimensional array, combinations thereof or any other now known orlater developed array. The transducer elements transduce betweenacoustic and electric energies. The transducer 14 connects with thetransmit beamformer 12 and the receive beamformer 16 through atransmit/receive switch, but separate connections may be used in otherembodiments.

The transmit beamformer 12 is shown separate from the receive beamformer16. Alternatively, the transmit and receive beamformers 12, 16 may beprovided with some or all components in common. Operating together oralone, the transmit and receive beamformers 12, 16 form beams ofacoustic energy for scanning a one, two, or three-dimensional region.Vector®, sector, linear or other scan formats may be used.

The transmit beamformer 12 is a processor, delay, filter, waveformgenerator, memory, phase rotator, digital-to-analog converter,amplifier, combinations thereof, or any other now known or laterdeveloped transmit beamformer components. In one embodiment, thetransmit beamformer 12 digitally generates envelope samples. Usingfiltering, delays, phase rotation, digital-to-analog conversion andamplification, the desired transmit waveform is generated. In otherembodiments, the transmit beamformer 12 includes switching pulsers orwaveform memories storing the waveforms to be transmitted. Othertransmit beamformers 12 may be used.

The transmit beamformer 12 is configured as a plurality of channels forgenerating electrical signals of a transmit waveform for each element ofa transmit aperture on the transducer 14. The waveforms are unipolar,bipolar, stepped, sinusoidal, or other waveforms of a desired centerfrequency or frequency band with one, multiple, or fractional number ofcycles. The waveforms have relative delay and/or phasing and amplitudefor focusing the acoustic energy. The transmit beamformer 12 includes acontroller for altering an aperture (e.g. the number of activeelements), an apodization profile (e.g., type or center of mass) acrossthe plurality of channels, a delay profile across the plurality ofchannels, a phase profile across the plurality of channels, centerfrequency, frequency band, waveform shape, number of cycles, coding, andcombinations thereof.

The transmit beamformer 12 is operable to transmit one or more transmitbeams of ultrasound energy substantially simultaneously. A transmit beamoriginates from the transducer 14 at a location in the transmitaperture. The transmit beam is formed along a scan line at any desiredangle. The acoustic energy is focused at a point along the scan line,but multiple points, line focus, no focus, or other spread may be used.The transmit beam substantially covers a wide region, such as beingdivergent, a plane wave, collimated, unfocussed, weakly focused, orfocused to cover multiple receive lines. Substantially accounts forsufficient acoustic energy to provide echoes and imaging above noise. Inone embodiment, the transmit beam is sufficiently wide to cover up to 64receive beams or scan lines distributed in a column (e.g., 8×8), a plane(1×64), or other arrangements (e.g., 4×16). By controlling theapodization, aperture, and delay profile, different size regions may bescanned with a given transmit beam.

The transmit beamformer 12 may generate multiple or split beams. Thesplit beams are formed for pulsed wave spectral Doppler estimation fortwo regions substantially simultaneously. In alternative embodiments, asingle transmit beam is formed for each transmit event.

For split beams, more than one transmit beam is generated substantiallysimultaneously. For example, a transmit beam is generated with a gratinglobe. The focus, apodization, aperture (e.g., discontinuous selection ofelements), or other characteristic is set to cause a grating lobe atsufficient amplitude for generating echoes above any noise. A highamplitude transmit beam may be steered at an angle away from normal tothe array to generate the grating lobe. Samples are received in responseto the primary beam and the grating lobe. As another example, thetransducer array is divided into two or more apertures. The separateapertures are used to form the different transmit beams. In anotherexample, frequency or other coding is used. For yet another example, thesame aperture is used to transmit multiple beams by combining delayedwaveforms for both beams at each element. Combinations of these examplesmay be provided.

The receive beamformer 16 is a preamplifier, filter, phase rotator,delay, summer, base band filter, processor, buffers, memory,combinations thereof, or other now known or later developed receivebeamformer component. Analog or digital receive beamformers capable ofreceiving one or more beams in response to a transmit event may be used.For example, the receive beamformer 16 has sufficient processing powerand/or hardware components to substantially simultaneously form 64 orother number of receive beams in response to a same transmit. Paralleland/or sequential processing may be used to form different beams.Parallel beamforming may be provided without storing ultrasound samplesfor each element for an entire receive event in a memory. Alternatively,a memory may be used to store the ultrasound samples for each element.

The receive beamformer 16 is configured into a plurality of channels forreceiving electrical signals representing echoes or acoustic energyimpinging on the transducer 14. A channel from each of the elements ofthe receive aperture within the transducer 14 connects to an amplifierfor applying apodization amplification. An analog-to-digital converterdigitizes the amplified echo signal. The digital radio frequencyreceived data is demodulated to a base band frequency. Any receivedelays, such as dynamic receive delays, and/or phase rotations are thenapplied by the amplifier and/or delay. A digital or analog summercombines data from different channels of the receive aperture to formone or a plurality of receive beams. The summer is a single summer orcascaded summer. The summer sums the relatively delayed and apodizedchannel information together to form a beam. In one embodiment, thebeamform summer is operable to sum in-phase and quadrature channel datain a complex manner such that phase information is maintained for theformed beam. Alternatively, the beamform summer sums data amplitudes orintensities without maintaining the phase information. Other receivebeamformation may be provided, such as with demodulation to anintermediate frequency band and/or analog-to-digital conversion at adifferent part of the channel.

For parallel receive operations, different delays, apodization, andsumming are provided for the different beams. For split ormulti-transmit beam, equal or different numbers of parallel beamformingare used for each beam. For example, two transmit beams are formed.Thirty two receive beams are formed for each of the two transmit beams.As another example, eight receive beams are formed from one transmitbeam and twenty four receive beams are formed from another transmitbeam.

Beamforming parameters including a receive aperture (e.g., the number ofelements and which elements used for receive processing), theapodization profile, a delay profile, a phase profile, imagingfrequency, inverse coding, and combinations thereof are applied to thereceive signals for receive beamforming. For example, relative delaysand amplitudes or apodization focus the acoustic energy along one ormore scan lines. A control processor controls the various beamformingparameters for receive beamformation.

One or more receive beams are generated in response to each transmitbeam. For example, up to 64 or other number of receive beams are formedin response to one transmit beam. Each receive beam is laterally and/orelevationally spaced in two or three-dimensions from other receivebeams, so samples are acquired for locations along different scan lines.

Acoustic echoes are received by the transducer 14 in response to thetransmit beam. The echoes are converted into electrical signals by thetransducer 14, and the receive beamformer 16 forms the receive beamsfrom the electrical signals. The receive beams are collinear, paralleland offset or nonparallel with the corresponding transmit beam. Thereceive beams may be adjusted to account for spatial two-waydifferences, such as adjusting the delay profile and/or amplitudedifferently for receive beams closer to the transmit beam center thanfor receive beams spaced further from the transmit beam center.Alternatively, a single receive beam is generated for each transmitbeam.

The receive beamformer 16 outputs data representing different spatiallocations of a scanned region. The receive beamformer 16 generatessamples at different depths along each receive beam. Using dynamicfocusing, samples are formed for different depths. By using differentreceive beams and scan lines, samples are formed from two- orthree-dimensional distribution of locations. The ultrasound data iscoherent (i.e., maintained phase information), but may includeincoherent data.

The image processor 18 includes a spectral Doppler processor and/orimaging detectors. In one embodiment, the image processor 18 is adigital signal processor or other device for applying a transform to thereceive beam data. A sequence of transmit and receive events isperformed over a period. A buffer or the memory 22 stores the receivebeamformed data from each transmit and receive event. Any pulserepetition interval may be used for the transmit beams. Any number oftransmit and receive events may be used for determining a spectrum, suchas three or more. The image processor 20 estimates a spectrum for eachof the different locations (e.g., each of the depths of each of thereceive beams in a region of interest). By applying a discrete or fastFourier transform, or other transform, to the ultrasound samples for thesame spatial location, the spectrum representing response from thelocation is determined. A histogram or data representing the energylevel at different frequencies for the period of time to acquire thesamples is obtained. FIG. 2 shows one example spectrum for a spatiallocation.

By repeating the process, the image processor 20 may obtain differentspectra for a given location at different times. Overlapping data may beused, such as calculating each spectrum with a moving window of selectedultrasound samples. Alternatively, each ultrasound sample is used for asingle period and spectrum.

A spectrum may be determined for each of a plurality of spatiallocations, such as for over 200 depths on each of 64 or other number ofreceive beams. The data for each location is transformed. The imageprocessor 18 may include a plurality of components for parallelprocessing or a single component for parallel or sequential estimation.

The image processor 18 may derive information from a given spectrum orfrom a plurality of spectra. In one embodiment, the image processor 18determines a clutter level, signal-to-noise ratio, maximum velocity,velocity range, and/or other characteristics. By determining a maximumvelocity or other characteristic of each spectrum, locations associatedwith motion or flow may be identified. An optimal location for theDoppler gate is identified.

The image processor 18 may include a B-mode detector for determiningintensity from samples acquired for spectral Doppler. The imageprocessor 18 may include a correlation processor or other color Dopplerdetector for determining average velocity, variance, and/or energy fromthe samples acquired for spectral Doppler. One or more filters, such asclutter, spatial or temporal filters may be provided.

The detector outputs incoherent image data. Additional processes, suchas filtering, interpolation, and/or scan conversion, may be provided bythe image processor 18.

A processor 21 is provided. The processor 21 may be part of the imageprocessor 18. The processor or processors used for estimation ordetection control the imaging and/or system 10. The processor 21 is ageneral processor, control processor, digital signal processor,application specific integrated circuit, field programmable gate array,graphics processing unit, analog circuit, digital circuit, combinationsthereof or other now known or later developed device for processing. Theprocessor 21 is configured by hardware, software, or both to performand/or cause performance of various acts, such as the acts discussedabove for FIG. 1.

The processor 21 is configured to set a location of the Doppler gate asa function of the sampled receive beams. The samples are acquired forspectral analysis at different locations. Without interleaving forseparate scanning in other modes, the samples for spectral Doppler mayalso be used for B-mode and/or F-mode detection. Alternatively, B-modeand F-mode detection are not performed and the location is set based onspectral analysis.

Using B-mode information, F-mode information, spectral Dopplerinformation, or combinations thereof, the location of the Doppler gatefor a given time or period is set. The location may be updated. To keepthe gate location at the desired feature, the setting of the gatelocation is repeated. The processor 21 repeats setting of the gatelocation.

The processor 21 may control the beamformers 12, 16. The beams andsampling may be centered on the Doppler gate location. As the gatelocation changes, the beamformers 12, 16 are controlled to change thesampling and beams. By positioning a center of the distribution oflocations, the processor 18 provides for more accurate setting of thegate location for later times. By positioning a center and/or focus ofthe transmit beam on the gate location, the signals for spectralanalysis may have better signal-to-noise ratio than for other locations.The center of the beam and focus represent regions of greater energy.The processor 21 controls the beamformers 12, 16 to change the lateral,elevation, and focus of the transmit and receive beams for centering oneof the transmit beams and/or the distribution of locations on the setDoppler gate location.

The image processor 18 generates display values as a function of thespectra estimated for the Doppler gate location. Display values includeintensity or other values to be converted for display, values providedto the display 20 (e.g., red, green, blue values), or analog valuesgenerated to operate the display 20. The display values may indicateintensity, hue, color, brightness, or other pixel characteristic. Forexample, the color is assigned as a function of one characteristic of aspectrum and the brightness is a function of another spectrumcharacteristic or other information. The display values are generatedfor a spectral strip display.

The display 18 is a CRT, monitor, LCD, plasma screen, projector or othernow known or later developed display for displaying an image responsiveto the display values. For a grey scale spectral Doppler image, a rangeof velocities with each velocity modulated as a function of energy isprovided as a function of time. The selected spectrum indicates thevelocity and energy information for a given time. The intensity of agiven pixel or pixel region represents energy where velocity is providedon the vertical scale and time provided on the horizontal scale. Otherimage configurations may be provided, including colorized spectralDoppler images.

The memory 22 stores buffered data, such as ultrasound samples forspectrum estimation. The memory 22 may store location information,B-mode information, F-mode information, spectra, characteristics ofspectra, display values or images, such as a CINE memory, or otherinformation.

In one embodiment, the memory 22 is a non-transitory computer readablestorage medium having stored therein data representing instructionsexecutable by the programmed processor 18 for positioning a Doppler gatein spectral Doppler ultrasound imaging. The instructions forimplementing the processes, methods and/or techniques discussed hereinare provided on computer-readable storage media or memories, such as acache, buffer, RAM, removable media, hard drive or other computerreadable storage media. Computer readable storage media include varioustypes of volatile and nonvolatile storage media. The functions, acts ortasks illustrated in the figures or described herein are executed inresponse to one or more sets of instructions stored in or on computerreadable storage media. The functions, acts or tasks are independent ofthe particular type of instructions set, storage media, processor orprocessing strategy and may be performed by software, hardware,integrated circuits, firmware, micro code and the like, operating aloneor in combination. Likewise, processing strategies may includemultiprocessing, multitasking, parallel processing and the like. In oneembodiment, the instructions are stored on a removable media device forreading by local or remote systems. In other embodiments, theinstructions are stored in a remote location for transfer through acomputer network or over telephone lines. In yet other embodiments, theinstructions are stored within a given computer, CPU, GPU or system.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

I(We) claim:
 1. A system for positioning a Doppler gate in spectralDoppler ultrasound imaging, the system comprising: a transmit beamformeroperable to transmit beams; a receive beamformer operable to form aplurality of spaced apart receive beams in response to each of thetransmit beams, each of the receive beams sampled at a plurality ofdepths; and a processor configured to set a location of the Doppler gateas a function of the sampled receive beams, to control the transmitbeamformer to center subsequent transmit beams on the location, and tocontrol the receive beamformer to center on the location a subsequentsample region defined by the sampling of the receive beams.
 2. Thesystem of claim 1 wherein the processor is configured to control thetransmit beamformer to center the transmit beams at the location.
 3. Thesystem of claim 1 wherein the processor is configured to track theDoppler gate location with B-mode information.
 4. The system of claim 3further comprising a spectral Doppler processor configured to estimate aspectrum for the Doppler gate location from signals for the trackedDoppler gate location.
 5. The system of claim 1 further comprising adisplay configured to display an image, the image being of a spectrum ofsignals at the Doppler gate.
 6. The system of claim 1 wherein theprocessor is configured to repeat the setting and controlling.
 7. Thesystem of claim 1 wherein the transmit beamformer is configured tosimultaneously transmit two of the beams from different apertures of atransducer array to laterally spaced apart scan lines and apply separatedelay patterns to the different apertures.
 8. The system of claim 1wherein the processor is configured to control the transmit beamformerto center by setting a focus of the subsequent transmit beams at thelocation.
 9. The system of claim 1 wherein the processor is configuredto control the receive beamformer to adjust a lateral and elevationposition of the receive beams to the location of the Doppler gate. 10.The system of claim 1 wherein the processor is configured to control thereceive beamformer to center a distribution of sample locations on thelocation of the Doppler gate.
 11. The system of claim 1 furthercomprising: a B-mode detector configured to detect intensity from singlesamples of the sample region for each of different times, and a spectralDoppler processor configured to estimate a spectrum for the locationfrom a plurality of the samples over time for that location, theplurality including the single samples.
 12. The system of claim 1wherein the processor is configured to determine a translation with agreatest similarity between B-mode information of different times forthe sample region and set the location as shifted by the translation.13. The system of claim 1 further comprising a display configured todisplay a spectral strip over time of spectra at the Doppler gate wherethe Doppler gate is at different tracked locations in the time.
 14. Thesystem of claim 1 further comprising a Doppler estimator configured toestimate F-mode information, and wherein the processor is configured toset as a function of the F-mode information.
 15. A non-transitorycomputer readable storage medium having stored therein data representinginstructions executable by a programmed processor for positioning aDoppler gate in spectral Doppler ultrasound imaging, the storage mediumcomprising instructions for: receiving signals over time for each aplurality of spaced apart locations; setting a Doppler gate location toone of the spaced apart locations as a function of the signals; updatinga distribution of the spaced apart locations as a function of the setDoppler gate location so that a center of the distribution is at the setDoppler gate location; and repeating the receiving with the distributioncentered at the set Doppler gate location, repeating the setting toanother Doppler gate location, and repeating the updating with thecenter of the distribution at the other Doppler gate location.
 16. Thenon-transitory computer readable storage medium of claim 15 whereinreceiving comprises receiving for a constellation of possible Dopplergate locations in response to multiple simultaneous transmit beams. 17.The non-transitory computer readable storage medium of claim 15 furthercomprising: updating a transmit beam position to be centered at the setDoppler gate location.
 18. The non-transitory computer readable storagemedium of claim 15 further comprising: detecting movement above athreshold; and initiating the setting in response to the movement abovethe threshold.
 19. The non-transitory computer readable storage mediumof claim 15 further comprising: generating B-mode information from pulsewave signals; tracking a region for the Doppler gate location with theB-mode information; and establishing the spaced apart locations based onthe tracking.
 20. A method for positioning a Doppler gate in spectralDoppler ultrasound imaging, the method comprising: receiving signalsover time for each a plurality of spaced apart locations in adistribution over a region of a patient; setting a Doppler gate locationto one of the spaced apart locations as a function of the signals, theone of the spaced apart locations being other than a center of thedistribution; updating the distribution of the spaced apart locations sothat the center of the distribution is at the set Doppler gate location;and repeating the receiving with the distribution centered at the setDoppler gate location.